Polysciences refrigerated circulating water bath

[JohnnyMalaria]

Hi,

I recently bought a VWR Polysciences 1162 refrigerated circulating water bath for $600 (instead of $1000s) (see attached).

I've noticed that when I am trying to maintain a temperature of 18degC (about 7degC below ambient) the compressor turns off every 10-20 minutes for about 5 minutes. This doesn't seem right. I don't know if there's a controller fault, low refrigerant or overheating. I have attached a temperature vs. time of day graph. The start of each blip is when the compressor switches off. There's a small initial rise because the heating continues. Eventually the compressor kicks back in.

My main concern is if this is okay behavior (I can live with temp control within 1degC but would prefer better) or if it is something that will damage the unit.

Any pointers to service manuals (generic or otherwise), advice etc will be most gratefully received  (web searches haven't been much use).

Thanks,

John.

[jmelson]

Does it have a heater?  If not, the only way to regulate temperature is starting/stopping the compressor.  We do have some baths here that run the compressor all the time, and regulate with a heater.  The heating can be turned on and off in a fraction of a second, so you get tighter temp control, at a cost of higher energy consumption.

Jon

[JohnnyMalaria]

Hi,

Yes, it does. The temperature should be maintained to within 0.1degC according to the specs for similar items (which this does when both the compressor and heating are on together). That's why I wonder if something's amiss and is a sign of impending failure.

John.

[KaneTW]

Considering the shape of the curve, it indeed looks like there's something wrong (it's basically flat except when it turns off and on again). Does the display show anything of value?

Try contacting VWR.

[JohnnyMalaria]

No, there's just the two green indicator lamps. The lower one shows when the compressor is running, the upper is for heating. Ordinarily, the compressor runs continuously and the heating pulses.

I have looked inside and it uses a small commercial Tecumseh hermetic compressor. I found the service manual. Now I know it was made on 29 March 2001 and uses R134a as the refrigerant. I have a set of pressure gauges for use with R134a heat pumps so at least I can test the refrigerant level.

[amyk]

Check the current draw of the compressor and make sure it's within spec; also worth checking whether it's turning off because the control board says so, or if it's the overload protection cycling it.

Low refrigerant will manifest as no or little cooling.

[JohnnyMalaria]

Well, I've made some progress.

I measured a variety of temperatures including the discharge line from the compressor to the evaporator coil. 80+ degC. If I set the unit to maintain 13degC then the problem goes away and I get control with +/-0.1degC for at least two hours. Raising the set temp to 18degC (i.e., nearer to ambient) does lead to the problem. Monitoring the discharge line shows that it cuts out at 101degC. Running at a lower set temp is actually a beneficial thing for my experiments.

[oventech]

An idea that might help. Wrap the bath with an insulating blanket to lessen the warmup of the bath from the ambient temp. The bath will see less ambient temp and hold temp better.